The present invention relates to a storage system, in particular to management of a hierarchical storage system.
Recently, the amount of data that is treated in the information systems has been increasing tremendously; therefore, the amount of storage needed by a company or organizations is also increasing. As with other sizeable business expense, it is desirable to reduce the cost of managing and maintaining a storage system. Hierarchical Storage Management (HSM) has been put forth to reduce such a cost. HSM system manages multiple kinds of storage devices, e.g., some are high performance and/or reliable devices that are relatively expensive and others are low performance devices that are less expensive.
In such a HSM system, important data are generally stored on the high performance and high cost storage while less important data are stored on the low performance and low cost storage. Since the importance of data tends to decrease over time, data are moved between the high performance storage to the low cost storage over time.
IBM Redbook SG-24-4877-03 “IBM Tivoli Storage Management Concepts”, pp. 151-158 discloses the concept of HSM function of IBM Tivoli Storage Manager for Storage Management (“TSM”), which is the storage management software of IBM. For example, TSM automatically migrates the least-recently used files to the lower cost storage media, and the “stub file” is placed in the original location of the migrated files. If user tries to access the migrated file, TSM retrieves the contents of the migrated file from the lower cost storage media based on the information in the stub file. This migration and retrieval operations are done by the IBM Tivoli storage Manager for Storage Management.
One problem of TSM is that the file migration operation is performed primarily by the TSM, i.e., the central processing unit (“CPU”) of a host computer. This may put a significant burden on the CPU. Another problem is that if TSM attempts to find a migrated file, it first finds the stub file based on the file/directory name information provided by the user, and then checks the contents of the stub file, and finally accesses the migrated file. That is, TSM performs two search operations: (1) stub file search and (2) migrate file search.
U.S. Pat. No. 6,237,063 discloses a method for exchanging the physical locations of the logical volumes, so that the workload of the physical disk drives can be equalized. The location exchange of each logical volume is performed transparently from the host computers.
One problem of the above method is that it exchanges the location only based on the access statistics of the logical volumes or physical disks. For instance, as a result of the exchange, important data may accidentally be migrated to the low-cost storage. Alternatively, unimportant/unused data may be migrated to the high-performance storage. Therefore, this method provides somewhat unpredictable data migration results.